With the increasing availability of lasers, including pulsed lasers which produce very high intensities of optical radiation in pulses of short duration such as 1 nanosecond or less, it has become possible to develop many different new optical applications which operate on a time scale ranging from less than 0.1 picoseconds or 0.0001 nanoseconds up to 1 nanosecond. Examples of these applications are high speed optical communication and computing, optical data recording, security devices, and high speed imaging. The very high intensities of the optical radiation in very short exposure times also creates a need for protective devices which can shield eyes and mechanical sensors from excessive optical radiation. U.S. Pat. No. 4,933,110 to Tucker describes an optical shield for protection from lasers.
In many of these laser-based applications, there is a need for optical devices which change their optical transmission or optical density properties depending on the presence or absence of the laser radiation. Optical density, as used herein, is defined as log.sub.10 (1/transmittance) for transmitted radiation. For example, with the protective devices, the optical transmission needs to reduce to an extremely low level, e.g., less than 0.01% for protecting the eyes of humans, at the wavelength of the incident laser radiation in a very short time before the cumulative optical radiation exposure causes damage to the eye, e.g., more than 0.5 microjoules/cm.sup.2. For example, in optical communications, typically involving the use of optical fibers, optical devices are used to modulate the laser signals at response times on the order of 1 picosecond.
Because of the extremely fast nature of these laser-based applications with time scales of less than a picosecond to 1 nanosecond, a variety of materials with different types of optical properties have been used to change the transmission properties of the laser radiation when passing through the devices of these applications. Materials which change their optical transmission properties upon undergoing a chemical change have not been suitable for these applications because of the long time scale of chemical reactions, e.g., 1000 nanoseconds or 1 microsecond up to seconds and minutes. This includes photochromic and conventional thermochromic materials, which undergo an optical absorption change due to a change in chemical structure when exposed to photons and heat, respectively. U.S. Pat. No. 5,091,984 to Kobayashi et al. describes an example of photochromic materials. U.S. Pat. No. 5,426,143 to de Wit et al. describes an example of conventional thermochromic materials. Another disadvantage of optical materials that depend on a chemical reaction to produce a useful optical change is that it is difficult to make them reversible over many thousands and more of optical changes. This is well known to be true for photochromic and thermochromic materials. Most high speed optical applications require very efficient reversibility because, by the nature of many repeat optical operations in 1 nanosecond or less time periods, they require millions and more of repeat optical changes during their useful life.
These requirements for very fast optical response times and for repeated reversibility have combined to make materials with a variety of nonlinear optical properties evaluated for use in these applications. These nonlinear optical materials do not typically undergo any chemical change during the optical change and are reversible in their change in optical properties. The optical nature of the change allows it to occur in typically less than 1 picosecond. Examples of the nonlinear optical changes are changes in optical transmission or absorption, changes in frequency of the laser radiation, and changes in the index of refraction. U.S. Pat. No. 5,472,786 to Pannell et al. describes optical switching materials based on changes in the index of refraction. Although these materials have found use in various high speed laser applications, many of them are inorganic materials and are difficult to fabricate into optical devices such as in a coated layer on a flexible substrate or in a plastic molded material. U.S. Pat. No. 5,406,407 to Wolff describes an optical switching device based on an inorganic material. Some of the nonlinear optical materials are difficult to fabricate into devices because their performance requires many extremely thin layers, e.g. dozens of layers only one or two molecular diameters thick for each layer, in order to provide an adequate nonlinear optical effect. Also, the magnitude of the optical changes with these nonlinear optical materials is often small, particularly for changes in optical transmission so that applications that would benefit from a large change in optical transmission, e.g., an optical shutter or eye and sensor protection devices, can not be readily done.
Therefore it is an object of this invention to provide an optical shutter which has a large change in optical transmission when exposed to a high intensity of photons in a very short period of time, such as 1 nanosecond or less.
It is a further object of this invention to provide an optical shutter for laser applications which is reversible and can be used repeatedly without degradation.
It is another object of this invention to provide an optical shutter that requires only 1 to 5 layers of materials and that can be fabricated using economical coating, plastic molding, and other conventional processes.
These and other objects of this invention will become more apparent from the following description of the invention.